The joinability of aluminum alloy and steel sheets using a self-piercing rivet was evaluated using a finite element simulation and experiment. Self-piercing riveting is a potential replacement of spot resistance welding generally used for steel sheets, because it is not easy to apply resistance welding to the joining of aluminum and steel sheets of vastly different melting points. To obtain optimum joining conditions, joining defects are categorized into (1) penetration through the lower sheet, (2) necking of the lower sheet and (3) separation of the sheets. The penetration, the necking and separation defects are caused by a small total thickness, a small thickness of the lower sheet and a large total thickness, respectively. The joining range for the combination of an upper sheet of steel and a lower sheet of aluminum is wider than that of the reverse combination. The joint strength is influenced by not only the deformation of the sheets and rivets but also the thickness and strength of the sheets.